1. Field of the Invention
This invention relates to a force detecting sensors and method for making such sensors and, more particularly, to cold-cathode microsensors.
2. Description of Related Art
Force detecting sensors are an integral component of a wide variety of control systems such as accelerometers, pressure sensors, tactile sensors and vibration sensors. In many applications, such as an accelerometer for spacecraft navigational systems, minimizing the weight and size of the force detecting sensor is critical. In addition, force sensors are preferably relatively low cost items produced by cost effective processes.
Force sensors typically include a movable member which is deflected when a force, e.g., the force of acceleration, is applied to the movable member. An electrical circuit senses the amount of displacement or change of position of the movable member and generates an output signal indicative of the applied force. Integrated circuit technology is used in the production of small scale, highly sensitive force sensors. A number of studies have been made in recent years in the field of cold-cathode solid state microsensors in which the movable member is used as an electrode and variations in current flow or field strength between the movable member and fixed electrode are used as a measure of displacement of the movable member. The so-called electron tunneling effect may be used to detect very small displacements.
Electron tunneling was first observed in the 1950s and since has been incorporated into numerous solid state structures. Electron tunneling occurs when potential is applied between conducting electrodes separated by a few atomic diameters within a vacuum environment and increases with decreasing electrode separation. A vacuum tunnel junction is one of the most sensitive position-detection mechanisms available, For example, a decrease in electrode separation in a vacuum tunnel junction of approximately 3 Angstroms produces a thousand-fold increase in electron tunnel current.
In an article by Kenny et al. entitled "A Micro Machine Silicon Electron Tunneling Sensor", IEEE, April 1990, p. 192, the authors disclose a prototype position-detection mechanism employing the electron tunneling effect to detect displacement. One described arrangement incorporates an aluminum mounting block to support a piezoelectric cantilever having a gold film electrode spaced apart from a gold, cold-cathode tunneling tip. This piezoelectric system is difficult and expensive to fabricate. In addition, the system is extremely sensitive to thermal drifts and hysteresis. Sensors having metal cathode emitters also suffer from the adsorption of work function raising impurities on the surface of the emitter. In addition, thermal mismatch between the metal cathode and the substrate can lead to cathode failure.
Kenny et al. further disclose a micro-machined Si wafer tunnel sensor composed of single crystals of Si. A folded cantilever spring of Si is created in a single Si wafer by etching. A gold film is evaporated onto the spring to make electrical contact to the tunneling electrode and a tunnel tip is formed by applying indium solder with an iron and then evaporating a gold film over it. According to the authors, the actual shape of the electrode in this structure is unimportant.
A theoretical study for field emission emitter array diodes is discussed by Lee et al. in "A Theoretical Study on Field Emission Array For Microsensors", IEEE Transactions on Electron Devices, Vol. 39, No. 2, p. 313, February 1992. The proposed structure disclosed in the theoretical study comprises a plurality of cone or wedge-shaped emitter arrays mounted below a single Si diaphragm.
What is not seen in the prior art of microsensors is a force detecting sensor which is practical to manufacture and not cost prohibitive, but which will supply the necessary sensitivity required for use in accelerometers, microsensors and vibration sensors.